Method and device for removing dents from sheet metal parts

ABSTRACT

Method of removing dents from sheet-metal parts, in particular from lacquered parts of an automobile body, wherein the metal part ( 3 ) in the region of a dent (B) is warmed locally in a substantially non-contact manner, in such a way that a mechanical tension gradient produced by the local warming causes the dent to spring back so as to restore the original shape of the metal part.

The invention relates to a method of removing dents from parts made ofsheet metal, in particular lacquered parts of an automobile body, aswell as an apparatus for implementing this method.

The removal of dents from pieces of sheet metal with the aim ofrestoring, as far as possible, an original (flat or curved) shape is atask that occurs in many branches of industry and the trades. Although alarge number of technical procedures and devices have been developed toaccomplish this task, some of which are commercially available, theproblem is still often solved by the manual efforts of trainedpersonnel. This entails considerable expense.

Probably the most economically important application of dent-removingmethods and apparatus is the repair of motor vehicles.

Hail damage to cars, in particular to new cars parked in the storageyards of automobile manufacturers, alone accounts for an annual averageof several hundred million DM in Germany. The dents caused by hailstonesare usually small and rounded, so that their shape and dimensions oftenallow them to be pressed out manually. However, somewhat largerindentations that cause a plastic deformation of the metal cannot beremoved in this way. To eliminate such relatively large dents variouskinds of apparatus have been proposed. These operate, for example, onthe basis of locally reduced pressure or on a magnetic principle, andare intended to restore the original shape of the affected piece ofmetal without elaborate dismantling procedures. Application of suchapparatus in some cases demands great experience and hence must becarried out by correspondingly highly paid workers, and neverthelessoften is not as successful as would be wished. In many cases, therefore,additional laborious lacquering work is inevitable.

The objective of the invention is thus to disclose a method and anapparatus for the removal of dents from pieces of sheet metal that aredistinguished in particular by low production and operating costs andhigh service value.

This objective is achieved with respect to its methodological aspect bya method with the characteristics given in claim 1 and with respect tothe apparatus by an arrangement with the characteristics given in claim11.

The invention includes the essential idea that to remove relativelysmall dents thermal energy is introduced into a piece of sheet metal ina narrowly circumscribed region thereof—specifically, the region of thedent—and the metal is caused to spring back into the original state bythe resulting mechanical tension gradient. Surprisingly, the inventorsdiscovered that this action can be achieved even with warming tocomparatively low temperatures, well below critical temperatures atwhich the customary surface coatings of sheet-metal parts (inparticular, the lacquer coating of an automobile body) undergo thermaldamage. This enables a very broad application of the proposed method,especially for eliminating small sites of damage to automobiles such asthe hailstone damage described above.

In a preferred embodiment of the method, the local warming is producedby targeted application of radiation, a substantial proportion of whichis in the near-infrared region, in particular, in the wavelength regionbetween 800 nm and 2 μm. In this embodiment the method can beimplemented with an apparatus that is particularly simple and economicalto manufacture and can be operated by workers with no specialexperience. By suitably adjusting the irradiation wavelength to thecharacteristics of the materials involved (the metal and whatevercoating is present), it becomes possible in particular to introduce theenergy substantially directly into the metal, while largely avoiding thecoating.

Alternatively, the dented metal part can be locally warmed by aninductive means or by a directed stream of hot air.

A crucial consideration is that the warming is done in a targeted andlimited manner, and in particular is substantially punctate in thecentral region and/or annular in the peripheral region of the dent thatis to be removed. As a result, the tension gradient that causes themetal to spring back into the original shape is built up in a controlledmanner. In the case of coated metal parts, the local warming is inparticular limited to a final temperature, which is low enough thatirreversible changes in the coating (e.g., a layer of lacquer) cannotyet occur. Even under this limiting condition, suitable preliminarysetting of the size and shape of the warming zone and, whereappropriate, of the rate of heating preserve sufficient degrees offreedom to enable indentations of various sizes and shapes to beeliminated from metal pieces made of various materials, in particularlacquered sheet steel.

Especially for repairing the latter, according to tests carried out bythe inventors it has proved sufficient to warm the region of the dent ina lacquered automobile body locally to a temperature in the rangebetween 100° C. and 200° C.

In an advantageous further development of the idea underlying theinvention, during the local warming the momentary shape of the metalpart is monitored, and when springing back into the original shape isdetected, the response is to terminate the warming process. This furtherdevelopment makes it possible partially to automate the method. Incombination with the above-mentioned temperature limitation, thisembodiment in particular enables the method to be carried out even bypersonnel with no special training and experience.

When the metal parts to be processed are of exactly the same kind andhave similar coatings—as is the case, for instance, when hail damage tomotor vehicles is being repaired—the above-mentioned temperaturelimitation can be achieved by a suitable design of the heating device,so that where appropriate it is possible to eliminate the need fortemperature measurement. It is somewhat more complicated for the methodto include the measurement and regulation of momentary temperature, butan apparatus capable of these functions can also be used for a widevariety of sheet metals and coatings.

Implementation of the method can be additionally facilitated with adesign in which the site of the local warming is marked by a visiblelight beam, and the heating device is directed towards the spot of lightthus produced on the metal part.

An apparatus for carrying out the method described above comprises inparticular a heating device that functions in a substantiallynon-contact manner, which advantageously comprises an adjustableheat-introduction region. Such a device can be optimally adapted toindentations of various sizes and to metal sheets that differ inthickness.

In an especially simple and economical embodiment the heating deviceoperates in the near-infrared region (NIR) and exhibits a predetermineddirectional characteristic for the emitted radiation. Such a heatingdevice preferably comprises an approximately punctate halogen lightsource, which is operated at a surface temperature of 2500 K or more, inparticular 2900 K or more.

In an embodiment that is preferred because of its versatility regardingpossible applications, the heating by NIR radiation has a variabledirectional characteristic, achieved in particular by an adjustableaperture or apertures and/or mechanical adjustment means—in particularto modify the position or shape of a reflector.

In accordance with the preferred further developments of the basic ideaof the method, as described above, the apparatus in particular comprisesa temperature-measurement device to monitor the surface temperature ofthe metal part in the warming region, which is advantageously connectedto a control input of the heating device so that a temperatureregulation (at least a temperature limitation) is produced in dependenceon the result of the temperature measurement. In an advantageousembodiment the temperature-measurement device incorporates a non-contactsensor, specifically a radiation pyrometer.

In another preferred embodiment of the proposed apparatus an aimingmeans is provided with which to direct the heating device towards theregion of a dent. This aiming means specifically comprises a radiatorfor visible light that is fixedly attached to the heating device and iseither self-focussing or provided with associated focussing means, so asto produce a narrowly circumscribed spot of marking light on the metalpart.

This marker enables the operator to position the heating deviceprecisely with respect to the dent.

An additional preferable feature is the provision of an opticalmeasuring device to monitor the momentary shape of the metal part, whichcan be constructed for instance as a laser-triangulation device. As aresult of the measurement of the surface shape of the sheet metal, inparticular this device sends out a “finished” signal as soon as itdetects that the indented area has sprung back so as to restore theoriginal state of the metal part. This signal can be used to turn offthe heating device directly and/or can be displayed visually to theoperator.

It is especially advantageous to use a single light source for both theaiming and the measuring device in combination, and in this regard it isparticularly advantageous to employ a laser radiation source, forexample an economical laser-diode device.

Advantages and useful features of the invention will additionally beapparent from the subordinate claims and from the following descriptionof exemplary embodiments with reference to the figures, wherein

FIG. 1 is a cross-sectional drawing of a dent-removal hood as the maincomponent of an apparatus to remove dents from sheet-metal partsaccording to one embodiment of the invention, and

FIG. 2 is a schematic representation of the essential components of anapparatus for dent removal according to one embodiment of the invention,in the form of a block diagram.

FIG. 1 shows a sketch of a hood 1 that is used to remove a small dent Bin a piece of sheet metal 3 that forms part of an automobile body and isprovided with surface coating of lacquer 3 a. The external shape of thehood 1 is determined bag a reflector 5 that is made of metal or at leastmetallized on its inner surface and has the shape of an ellipsoid ofrevolution, the end of which has been cut off and which has the twofocal points F1 and F2. Disposed circumferentially at the edge of thereflector 5 is a rubber ring 7, by way of which the reflector can beharmlessly set onto the body metal 3. The reflector 5 also comprises anopening 5 a through which the region of the dent B in the body metal 3can be observed when the hood 1 has been put into place. Finally, aplastic handle 9 is attached to the reflector so that the hood 1 can bemanipulated easily and with no danger even during operation.

The dent-removal hood is connected to the alternating-current mainssupply by way of a current cable 11, and to a measurement and controlsystem (not shown here) by way of a data line 13. In the interior of thehood, screwed to the reflector 5, is a plastic carrier 15 that houses atransformer (not shown) and from which a lamp socket extends towards thefocal point F1; two extension arms 15 b, 15 c are also disposed on thecarrier 15, to hold additional components described below. In the lampsocket 15 a is seated a halogen lamp 17, for example a Xe or Kr lamp,which is operated in such a way that it has a surface temperature of ca.2900 K and thus emits radiation, a substantial proportion of which is inthe near-infrared (NIR) region between 800 nm and 2 μm. The halogen lamp17 is positioned at the focal point F1, so that the NIR radiation itemits is largely reflected by the reflector 5, which is shaped like anellipsoid of revolution, in such a way that the radiation converges atthe focal point F2 and hence is reflected into the center of the dent Bin the body metal 3.

The extension arm 15 b of the plastic carrier 15 supports, first, alaser diode provided with a simple projection lens, which is sopositioned that the radiation it emits is directed towards the focalpoint F2. Also mounted on the extension arm 15 b is a radiationpyrometer element 21, which is likewise directed towards the focal pointF2 and serves to detect the surface temperature of the car-body metal 3in the region of the dent B. The extension arm 15 c bears aphotodetector arrangement 23, which is so constructed (e.g., as a simpleCCD array) and positioned that it detects the radiation from the laserdiode 19 that is reflected from the metal sheet 3 in different ways,depending on the state of the metal in the region of the dent B. Thephotodetector arrangement 23 can, in particular, be constructed so thatthe reflected radiation originating in the laser diode 19 reaches itonly when the configuration of the body metal is undisturbed (dashedline in drawing), or so that the radiation reflected from the region ofthe dent B reaches it at a different position than the radiationreflected from the undisturbed metal surface (with dent removed). Thusthe photodetector arrangement 23, in cooperation with the laser diode19, serves as a simple laser-triangulation device for measuring thesurface configuration of the body metal 3 in the region of the dent B;this is discussed further below. The laser diode 19 additionally servesto mark the focal point F2 and hence the site of maximal energy input ofthe NIR radiation from the halogen lamp 17, and enables an operator toposition the hood 1 accordingly by observing, through the opening 5 a inthe reflector 5, the spot of light produced on the body metal 3 by thelaser diode 19.

The hood 1 is operated as follows: first it is set onto the body metal 3so as to cover the dent B, then the laser diode 19 is turned on and thehood is shifted on the car body so that the light from the diode forms aspot centered in the dent B, and finally the region of the dent B iswarmed by actuating a mains switch (not shown) so that the halogen lamp17 is turned on. During the irradiation, the radiation-pyrometer elementis used to detect the temperature of the body metal 3 in the region ofthe dent B, and the photodetector arrangement 23 is used to detect itsshape; the halogen lamp 17 is turned off when the dent is found to havesprung back, restoring the original shape of the body metal, or when aprespecified permissible limiting temperature has been reached.

In the embodiment of the apparatus for implementing this method inaccordance with the invention as shown in FIG. 1, which is mechanicallyvery simple and inexpensive, the size and shape of the zone of bodymetal that is warmed can be modified by tilting the hood or raising itslightly; for this purpose more refined (and correspondingly moreelaborate and costly) embodiments include, for example,height-adjustment means and/or an adjustable aperture to block out partof the light beam from the halogen lamp at the irradiated site. Suchembodiments are within the scope of those skilled in the art and henceare not described in greater detail here.

In FIG. 2 the functional arrangement of the measurement and controlelements of a dent-removing apparatus 100 is shown schematically as ablock diagram. The dent-removing apparatus 100 is connected to thealternating-current mains, and from the mains voltage a power-supplyunit 101 draws and stabilizes the voltage needed to operate the halogenlamp 17. This operating voltage is sent from the power-supply unit 101to the lamp 17 by way of a power-control stage 103, which comprises twocontrol inputs 103 a, 103 b. The power-supply unit 101 likewise suppliesthe laser diode 19, in this case by way of a rectifier stage 105 and aseparate switch 107.

Two other components are also supplied by the power-supply unit 101: atemperature-measurement stage 109 connected on its input side to theradiation-pyrometer element 21 and at its output both to the controlinput 103 b of the switching and power-control stage 103 and to adisplay unit 111; and also a surface-geometry evaluation unit 113, whichat its input side is connected to the photodetector arrangement 23. Atits output side, the surface-geometry evaluation unit is connected tothe control input 103 a of the switching and power-control stage 103 aswell as to the display unit 111.

The function or the dent-removing apparatus 100 will already be to agreat extent apparent from the above explanation of the invention. Atthis juncture, therefore, it will merely be pointed out in summary thatby way of the radiation-pyrometer element 21, which detects the surfacetemperature of the car-body metal 3 (FIG. 1), the temperate-measurement,109 and the switching and power-control stage 103, to which the halogenlamp 17 is connected, a temperature regulation is achieved, at least inthe sense of a limitation of the surface temperature to a permissiblemaximal value at which the lacquered surface 3 is certain not to bedamaged. This is accomplished by reducing the radiation output when saidmaximal temperature is approached, or by transiently turning off thehalogen lamp 17. Complete switching off of the halogen lamp 17 isbrought about by the photodetector arrangement 23 and thesurface-geometry evaluation unit 113 as well as by the control inputs103 a of the switching and power-control stage 103, as soon as aprespecified signal processing in the surface-geometry evaluation unitsignals the result that a significant change in the geometry of thesheet metal has occurred in the region of the dent, in the sense thatthe metal has sprung back into the original, undistorted shape. Inparallel to these control or regulation processes, the operator isinformed about the actual surface temperature, by way of the displayunit 111, and also, for example by a signal lamp, about the fact thatthe original shape of the metal has been restored, i.e. that thedent-removing process has been successful.

The invention can be put into practice not only by the embodimentdescribed above, but also by a large number of modifications thereof.

For example, the reflector geometry of the mechanical-optical part ofthe dent-removing apparatus, termed “dent-removal hood” above, isvariable just as is the internal construction. In particular, of thefunctional components mentioned above, individual ones can be omitted inthe interest of further simplification (and hence reduced cost) of theapparatus: for instance (given that the radiation source isappropriately dimensioned) the means for temperature measurement or themeans for detecting the surface geometry of the body metal. Instead ofusing a laser diode to mark the region from which the dent is to beremoved, another light source can be provided, as long as it hasfocussing means suitable for producing a clearly delimited light spot onthe surface of the metal.

LIST OF REFERENCE NUMERALS

1 Dent-removal hood

3 Metal of motor-vehicle body

3 a Surface layer of lacquer

5 Reflector

5 a Opening

7 Rubber ring

9 Plastic handle

11 Current cable

13 Data line

15 Plastic carrier

15 a Lamp socket

15 b, 15 c Extension arm

17 Halogen lamp

19 Laser diode

21 Radiation-pyrometer element

23 Photodetector arrangement

100 Dent-removing apparatus

101 Power-supply unit

103 Switching and power-control stage

103 a, 103 b Control input

105 Rectifier stage

107 Switch

109 Temperature-measurement stage

11 Display unit

113 Surface-geometry evaluation unit

F1, F2 Focal point

B Dent

What is claimed is:
 1. Method of removing dents from sheet-metal parts,in particular from lacquered parts of an automobile body, wherein themetal part (3) is locally warmed in the region of a dent (B) in asubstantially non-contact manner, such that a mechanical tensiongradient induced by the local warming causes the dent to spring back soas to restore the original shape of the metal part, characterized inthat the local warming is carried out by the directed application ofradiation, a substantial proportion of which is in the near-infraredregion, in particular in the wavelength region between 800 nm and 2 μm.2. Method according to claim 1, characterized in that the local warmingis substantially punctate in the central region and/or annular in theperipheral region of the dent.
 3. Method according to claim 1,characterized in that the local warming is limited to a finaltemperature below a critical temperature at which irreversiblealterations would be produced in a coating (3 a) of the metal part (3),in particular a layer of lacquer.
 4. Method according to claim 1,characterized in that the local warming, in particular in the case of alacquered metal part of a car body, is carried out up to a finaltemperature in the region between 100° Celsius and 200° Celsius. 5.Method according to claim 1, characterized in that during the localwarming the momentary shape of the metal part (3) in the region of thedent (B) is monitored and when it is detected that the metal has springback into its original shape, the response is to terminate the warming.6. Method according to claim 1, characterized in that the during thelocal warming the temperature at the site of warming is measured. 7.Method according to claim 1, characterized in that the region of a dent(B) is marked as the site for local warming by means of a visible lightbeam.
 8. Apparatus for implementing the method according to claim 1,characterized by a heating device (5, 17, 101, 103) that operates in thenear-infrared region, in particular in the wavelength range between 800nm and 2 μm, and in particular has an adjustable heat-introductionregion and/or adjustable heat output, for the substantially non-contactlocal warming of the metal part (3).
 9. Apparatus according to claim 8,characterized in that the heating device (5, 17, 101, 103) isconstructed as an NIR-radiation heater with predetermined directionalcharacteristic, comprising in particular an approximately punctatehalogen lamp (17) that is operated at a surface temperature of 2500 K orhigher.
 10. Apparatus according to claim 9, characterized in that theNIR-radiation heater has a directional characteristic that can bealtered, in particular by at least one aperture and/or mechanicaldisplacement means.
 11. Apparatus according to claim 8, characterized bya temperature-measurement device (21, 109) to detect the temperature ofthe metal part (3) in the warming region (B), which in particular isconnected to a control input (103 b) of the heating device (103) in sucha way that by controlling the heating device in response to the outputsignal from the temperature-measurement device, the temperature in thewarming region is at least limited.
 12. Apparatus according to claim 11,characterized in that the temperature-measurement device operates in anon-contact manner and in particular comprises a radiation pyrometer(21).
 13. Apparatus according to claim 8, characterized by an aimingdevice (19) to direct the heating device (5, 17) towards the region ofthe dent (B), such that the aiming device in particular comprises aradiator for focused visible light that is fixedly attached to theheating device.
 14. Apparatus according to claim 8, characterized by anoptical measurement device (19, 23, 113) to detect the momentary shapeof the metal part (3) in the region of the dent (B).
 15. Apparatusaccording to claim 13, characterized in that the aiming and/or themeasurement device comprises a laser-radiation source (19) that inparticular serves as a common source of radiation for the aiming and themeasurement device.